The invention is based on a priority application EP 07290531.8 which is hereby incorporated by reference.
The invention relates to an optical network having a tree-like structure with a main line and a plurality of branches, at least two of the branches comprising a monitoring unit for upstream signalling to the main line, to a monitoring unit for generating a periodic upstream signal in such an optical network, and to a monitoring method for such an optical network.
In order to improve the quality of service in optical networks, in particular Passive Optical Networks (PONs) with a tree-like structure, continuous monitoring techniques detecting and localizing faults are of special interest, as they are expected to provide substantial cost savings to telecommunications companies operating these networks.
EP 1 578 038 A1 discloses an optical network which comprises an optical monitoring termination (OMT) at a head end of a main line of the network and at least one optical monitoring unit (OMU) connected to a branch of the network. The OMT and the OMU are arranged for communicating with each other using a dedicated monitoring wavelength which is extracted from the optical path between the OMT and the OMU using a wavelength-selective device, in particular a WDM (Wavelength-Division-Multiplex) coupler. The OMU comprises a reflective optical element which is switchable, i.e. in its “on” state reflects light at the extracted monitoring wavelength back to the optical path via the WDM coupler.
In the article “High Spatial Resolution PON Measurement Using an OTDR Enhanced with a Dead-zone-free Signal Analysis Method” by N. Araki et al., Optical Fiber Measurements, Technical Digest, 2004, Pages 69-72, a high spatial resolution OTDR measurement method for PONs is described which uses optical filters such as fiber Bragg gratings (FBGs) which are installed at the subscriber end of optical fiber lines of the PON and which allow a communication light to pass but reflects a test light. Using the signals reflected from the FBG filter, signal analysis is possible also in an attenuation dead zone just behind the Fresnel reflection of an optical splitter in the network.
The article “A New Fault-Identification Method Using a Dichroic Reflective Optical Filter in Optical Subscriber Loops” by H. Takasugi et al., Journal of Lightwave Technology, Vol. 11, No. 2, 1993, pp. 351-357 describes a method with which faults in an optical fiber line of an optical network can be identified by inserting a dichroic filter at the end of the fiber line, the filter reflecting light at a test wavelength and allowing light at a communication wavelength to pass. The level and distance of reflection from the filter are measured with an optical time domain reflectometer (OTDR) from the central office of the network.
In the approaches as described above, a high-power light source is required which may induce extra noise contributions to the data signal. Moreover, expensive equipment (high resolution OTDR or tunable laser source) is required for detection of the reflected signal at the central office.